Blood pressure measurement devices, also referred to as sphygmomanometers, of the type commonly used to measure arterial blood pressure, include an inflatable sleeve, commonly referred to as a cuff, adapted to fit around a limb, e.g. an arm or leg, of a patient. The cuff includes an interior chamber that is in fluid communication with a device for selectively inflating the interior chamber of the cuff with pressurized air. A gage is operatively connected in fluid communication with the interior chamber of the cuff for monitoring the air pressure within the cuff. A bleed valve is also operatively connected in fluid communication with the interior chamber to permit selective depressurizing of the interior chamber when it is desired to deflate the cuff.
In a typical conventional manual sphygmomanometer, the interior chamber of the cuff is connected through a length of flexible tubing to a pneumatic bulb. In operation, the cuff is fitted, e.g. wrapped, about the arm of the patient and, once so positioned, the cuff is inflated by squeezing the pneumatic bulb to force air through the tubing into the interior chamber of the cuff. Once the interior chamber of the cuff has been inflated to a desired level in excess of the patient's anticipated systolic blood pressure, as indicated on the pressure gage, the cuff is deflated by opening the bleed valve to allow the pressurized air within the interior chamber of the cuff to vent slowly to atmosphere. A stethoscope is positioned under the cuff and over the patient's brachial artery to monitor the patient's arterial pulses as the cuff deflates, thereby allowing the systolic and diastolic blood pressures to be determined by listening for the Korotkoff sounds.
The systolic and diastolic blood pressure can also be measured oscillometrically by detecting the minute changes in the cuff pressure due to flow of blood through the brachial artery. This oscillometric measurement also utilizes an inflatable cuff and generally employs one or more pressure sensing devices, such as a transducer, to monitor the pressure within the interior chamber of the cuff. The transducer monitors both the average pressure in the cuff and the minute changes in the cuff pressure due to flow in the patient's artery as the cuff deflates. Electronic circuitry is provided that processes the signals from the pressure-sensing device and determines the systolic and diastolic blood pressures. A motor driven pump is usually provided to inflate the cuff. However, the inflation can be produced via a pneumatic bulb. Typically, a digital display is provided for displaying the systolic and diastolic blood pressures.
To obtain accurate measurements, it is necessary to deflate the inflated cuff at a relatively constant rate in the range of about 2 to about 3 millimeters mercury (2-3 mmHg) per second or about 2 to about 3 millimeters mercury (2-3 mmHg) per heartbeat. Maintaining a relatively constant bleed flow rate has been a problem when using many prior art sphygmomanometers, particularly when used by untrained personnel. For example in conventional manual sphygmomanometers, a typical vent valve has an air passage through which the flow of air may be adjusted by selectively restricting the flow area by manipulation of a thumbscrew. The thumbscrew is tightened down to fully close the vent valve passage when the cuff is being inflated. With the cuff inflated to the desired starting pressure, the user manually turns the thumbscrew to slightly open the vent valve passage to initiate deflation of the cuff. The user also monitors the patient's artery using a stethoscope to listen for the aforementioned Korotkoff sounds to detect when the systolic pressure and the diastolic pressure have been reached. As the cuff deflates, the user should follow the decrease in cuff pressure as registered on the pressure gauge and should continually adjust the vent thumb screw to increase the flow area through the vent valve passage as the pressure in the cuff decreases in an attempt to maintain a nearly constant bleed rate. The user should maintain this rate within the desired rate range, while at the same time continuing to concentrate on listening for the characteristic Korotkoff sounds to detect the systolic and diastolic blood pressures. It is difficult to manually maintain the desired linear pressure decrease as the cuff deflates by adjusting a thumbscrew even under ideal conditions. It is even more difficult to do so when attempting to listen for the arterial sounds. Once the cuff pressure has dropped to the diastolic pressure point, the user must then further adjust the thumbscrew to fully open the vent passage, thereby providing a rapid final deflation of the cuff to ensure patient comfort.
An electronic blood pressure measurement apparatus can be designed with linear cuff pressure decrease. Such an apparatus may comprise a valve that vents the interior chamber of the inflated cuff through a port whose flow area is controlled electronically. To deflate the cuff, the controller selectively opens the valve. This selective opening is needed to compensate for the airflow behavior of a fixed area port. With a fixed area port, the vent flow rate varies as a function of the pressure differential across the port at any given time in the venting process. As the pressure within the interior chamber continuously decreases during the deflation process, the pressure differential—that is, the difference between the air pressure within the interior chamber of the cuff and ambient pressure—also continuously decreases. Therefore, since the pressure differential across the vent port is continuously decreasing, the flow rate does not remain relatively constant to provide the desired 2-3 mmHg/sec rate during the deflation process, but rather continuously decreases.
It is well known in the art that the inflated volume of the pressure chamber of the cuff affects the bleed rate through a conventional fixed area orifice bleed valve. For example, a cuff for a large adult typically has a pressure chamber having a length of approximately 27 centimeters and a width of approximately 12.5 centimeters, while a cuff for a small child typically has a pressure chamber having a length of approximately 13 centimeters and a width of approximately 5 centimeters. The fact that the inflated volume of the adult cuff is many times as great as the inflated volume of the child cuff has a substantial impact upon the rate of pressure decrease in the respective cuffs for deflation through a bleed valve having a fixed flow area opening. The larger volume cuff exhibits a slower pressure decrease rate at a given inflation pressure than the smaller volume cuff exhibits. As discussed above, this pressure decrease rate does not remain constant as the cuff deflates, but rather decreases as the cuff deflates due to the drop in pressure within the cuff.
In U.S. Pat. No. 4,587,974, Link discloses a pressurizing and depressurizing design for a blood pressure cuff with the objective of controlling the cuff inflation or deflation process at a substantially linear rate independent of the size of the cuff in use. In a preferred arrangement, the disclosed device includes a housing defining an internal volume divided by a wall supporting a flexible diaphragm. This division produces a control chamber and an active chamber, the wall and diaphragm forming a boundary therebetween. The active chamber is in pneumatic communication with the blood pressure cuff and with a vent passage to ambient surroundings. The control chamber is in pneumatic communication with the active chamber via an opening in the division wall, the opening forming pneumatic restrictor. The flexible diaphragm moves in response to the pressure differential between the active chamber and the control chamber to selectively open and close the outlet from the active chamber to the vent passage. In depressurizing the cuff, air from the cuff passes into the active chamber causing the diaphragm to flex away from the vent passage and toward the control chamber, thereby opening the vent passage to allow air to vent from the active chamber to ambient surroundings. Simultaneously, air passes between the control chamber and the active chamber through the restrictor opening between the chambers so as to rebalance the pressures between the chambers. In this manner, the diaphragm fluctuates to and fro relative to the outlet to the vent passageway so as to continuously open and close the vent passageway. This controls the rate at which the cuff deflates through the active chamber to produce a substantially linear rate determined by the flexing characteristic of the diaphragm.
However, the device disclosed in U.S. Pat. No. 4,587,974 does not provide for the rapid deflation of the cuff directly to ambient pressure for patient comfort once the diastolic pressure point has been reached. Additionally, when the vent valve of the device is opened at the beginning of the deflation process, an abrupt drop in cuff pressure can initially occur until the diaphragm responds sufficiently to cause closure of the vent port. Such an abrupt decrease can be relatively large, in particular for small cuffs, and disruptive of the blood pressure measurement process. Further, after this abrupt pressure decrease, a relatively large differential pressure may exist across the diaphragm while the air flows from the control chamber through the flow restrictor and into the active vent chamber to reduce the pressure differential. The deflation process can not start properly until this pressure balancing is completed. Once the deflation process does begin, the initial deflation of the cuff proceeds slowly until the eventual linear steady state rate is achieved.
In the device disclosed in the U.S. Pat. No. 4,587,974, the time required to the pressurize the system is lengthened because air passing into the control chamber must first pass through the flow restrictor, which causes a time delay between the time the cuff is pressurized and the control chamber is pressurized to cuff pressure. If the user doesn't wait until the pressure in the control volume reaches that in the cuff, there is an abrupt decrease in cuff pressure (which can be large for small cuffs) when the closure valve is opened to start the cuff deflation process. Also, the cuff pressure backs off while the pressure within the control chamber of the bleed valve equalizes with the cuff pressure, resulting in a decrease in cuff pressure, which may be large for small cuffs, requiring additional pumping action to return the cuff pressure to the desired starting pressure level.
The aforementioned deficiencies complicate the design of the device. For example, to minimize the aforementioned effects, the diaphragm must be selected to have low inertia. The diaphragm must also not require a high pressure differential across it to move the valve stem relative to the outlet vent opening through the operating range. The diaphragm must also provide sufficient flow area at the outlet port so that rapid deflation of large cuffs is possible.